The 3rd Generation Partnership Project (3GPP) started a Release 13 Long Term Evolution Advanced (LTE-Advanced) study item, Licensed Assisted Access (LAA), aiming to use the unlicensed spectrum, on which WiFi is currently deployed. It is observed that LTE significantly impacts WiFi performance in LTE-WiFi coexistence case, if current LTE functionalities are assumed. One major reason is that WiFi follows Listen-Before-Talk (LBT) principle, which specifies that a WiFi Node can only start transmitting after it has performed Clear Channel Assessment (CCA) and measured that the channel is idle, while a legacy LTE Node does not perform CCA and may transmit continuously. The main problem for LTE Release 13 LAA is how to achieve fair and effective coexistence with Wi-Fi, and among LAA networks deployed by different operators. To ensure fair co-existence with WiFi, LTE needs to be modified to also support LBT on the unlicensed spectrum band.
To ensure fair co-existence with WiFi, it is agreed for LAA to support LBT and discontinuous transmission as well as limited maximum transmission duration on a carrier in the unlicensed spectrum band. The LAA eNodeB can only start transmission when the channel is clear as measured by Clear Channel Assessment (CCA). After a transmission of limited maximum duration, the LAA eNodeB needs to release the channel and perform CCA again to use the channel, resulting in opportunistic transmission with maximum transmission time of around 13 ms for LBE (Load Based Equipment) and 10 ms for Frame Based Equipment (FBE).
For LBE, CCA is minimum 20 μs, extended CCA (eCCA) duration is a random factor N multiplied by the CCA time, where N is randomly selected in the range 1 . . . q every time, q=4 . . . 32, and Channel Occupancy Time is <=(13/32)×q ms. For FBE, CCA is minimum 20 μs and performed in the end of IDLE period, Channel Occupancy Time is 1 ms at minimum and 10 ms at maximum, IDLE period is Minimum 5% of channel occupancy time and Fixed Frame Period=Channel Occupancy Time+IDLE Period.
For LBE, the CCA may happen at any time and the CCA success may happen at any time accordingly. One option is that at least for LBE, some signal(s) can be transmitted by eNodeB between the time eNodeB is permitted to transmit and the start of data transmission, at least to reserve the channel. The starting time of the signals is immediately after CCA success and potentially does not fit into the orthogonal frequency-division multiplexing (OFDM) symbol timing, resulting in a fractional OFDM symbol with variable length from zero to one complete OFDM symbol.
For FBE, the downlink (DL) transmission can only happen at the start of the fixed frame period. Similarly, in case the starting time of the fixed frame period is not the OFDM symbol boundary, a fractional OFDM symbol to be transmitted.
Once the LAA eNodeB measures the channel as clear, after it transmits a potential fractional OFDM symbol to reserve the channel, it may transmit a preamble for time- and frequency synchronization followed by the downlink data. The user equipment (UE) gets synchronized to the LAA eNodeB based on the aperiodically transmitted preamble and is able to demodulate data immediately after the preamble. The preamble may contain a fractional OFDM symbol and at least one complete OFDM symbol, or may contain fractional OFDM symbol only, or may contain complete OFDM symbols only.
One conventional solution is to generate the fractional OFDM symbol from some pre-defined sequence, e.g. random sequence, dummy sequence or Zadoff-Chu (ZC) sequence. Specially, the LAA eNodeB generates the OFDM symbol from some pre-defined sequence, e.g. random sequence, dummy sequence or Zadoff-Chu (ZC) sequence, and then transmits the fractional OFDM symbol by using a part of the OFDM symbol such that it equals the duration of the fractional OFDM symbol.
OFDM spectral efficiency is also affected by out-of-band emission, which creates interference by the power emission of the OFDM signal. OFDM is known to have a rather slow decay of the spectral side lobes, which necessitates the transmitter to perform one or several measures to control the out-of-band emissions, e.g., transmit filtering, windowing or pulse-shaping. These methods are used to fit the spectral content of the signal within the limits given by spectral masks, adjacent-channel-leakage ratios and similar Out-Of-Band (OOB) emission requirements. Let the transmit signal on each single subcarrier represented by s(t), the Power Spectrum Density (PSD) of s(t) can be represented by:
            P      ⁢                          ⁢      S      ⁢                          ⁢              D        ⁡                  (          f          )                      =                  A        2            ⁢                                    T            0                    ⁡                      (                                          sin                ⁡                                  (                                      π                    ⁢                                                                                  ⁢                                          fT                      0                                                        )                                                            π                ⁢                                                                  ⁢                                  fT                  0                                                      )                          2              ,where A denotes the signal amplitude and T0 is the complete symbol duration, which consists of the sum of useful symbol duration TU and guard interval TG during which the cyclic prefix (CP) is transmitted to overcome Inter Symbol Interference (ISI), where the cyclic prefix refers to cyclically extending the symbol. It can be observed that the PSD of an OFDM subcarrier is fulfilling a sinc function which is featured by the main lobe of the largest power is of a frequency range equal to 1/T0, and the sidelobe is getting weaker and weaker with the frequency offset increasing and each sidelobe is also of a frequency range equal to 1/T0. Therefore larger T0 may imply the OFDM subcarrier energy is more concentrated in its allocated frequency and less power emission.
If the modulation symbols on the different subcarriers are uncorrelated, the PSD of the OFDM signal comprising N subcarriers can be expressed as:
      P    ⁢                  ⁢    S    ⁢                  ⁢          D      ⁡              (        f        )              =            ∑              k        =        0                    N        -        1              ⁢                  ⁢                  A        2            ⁢                                                  T              0                        ⁡                          (                                                sin                  ⁢                                                                          ⁢                  π                  ⁢                                                                          ⁢                                      (                                          f                      -                                              f                        k                                                              )                                    ⁢                                      T                    0                                                                    π                  ⁢                                                                          ⁢                                      (                                          f                      -                                              f                        k                                                              )                                    ⁢                                      T                    0                                                              )                                2                .            
As the symbol duration of the fractional OFDM symbol T0_F is shorter than the normal complete OFDM symbol duration T0, the power emission may be increased where it is assumed that the bandwidth is 20 MHz, the eNodeB transmission power is 36 dBm, the FFT size is 2048, and the cyclic prefix length is 144/(15000*2048) second. It can be observed that the power emission is increased if the fractional OFDM symbol duration is of ¾ useful symbol duration (¾ TU), and is further increased if the fraction OFDM symbol is reduced to ½ useful symbol duration (½ TU), or to ¼ useful symbol duration (¼ TU).
The increased out-of-band emissions due to fractional OFDM symbols will reduce the performance of the wireless communication system. It is also non-trivial to provide means in the transmitter for controlling the spectral emissions. In a typical OFDM system, the OFDM symbols have the same (or very similar) duration. Therefore, the transmitter can implement a transmit filter based on a typical OFDM symbol duration. Such filters are typically implemented in hardware as the filter coefficients may not to be changed dynamically. However, with fractional OFDM symbols, which may have varying length depending on the CCA process, the transmit filter design would be more complicated, allowing dynamic duration OFDM symbols, which will increase the cost of the filter.
In addition, during the fractional OFDM symbol period, there would be other communication devices performing CCA measurements on the adjacent bands, e.g. LAA or WiFi devices. If there is an increase of OOB emission due to reduced symbol duration, this may introduce more severe interference to other LAA Nodes, resulting in a smaller probability of a successfully reserving the channel, further resulting in unnecessary blocking of the transmissions on the adjacent bands.
One disadvantage of conventional solutions is the additional complexity due to generation of OFDM symbol from some pre-defined sequence, e.g. random sequence, dummy sequence or Zadoff-Chu (ZC) sequence. This may either result in FFT processing and OFDM modulation process for each fractional OFDM symbol transmission, or additional resource to store the pre-generated OFDM symbols samples.
A further disadvantage of conventional solutions is that it is not possible to generate arbitrarily short OFDM symbols. For example, in the LTE eNodeB, there could be a transient period of 17 microseconds when changing from an OFF state (i.e., no transmit power) to an ON state (i.e., full transmit power).
Another disadvantage of conventional solutions is increased OOB emission due to reduced symbol duration of a fractional OFDM symbol.